Each matrix must have identical row and column names corresponding to the levels
of the associated random effect. Alignment with the data is checked internally
within LMMsolve.
The function performs basic validation:
precisionMatrices must be a named list.
Each matrix must be square with identical row and column names.
Row and column names are used later to align matrices with factor levels
in the data.
No reordering or alignment with the data is performed at this stage. This is
handled internally by LMMsolve.